Problem: The arithmetic sequence $(a_i)$ is defined by the formula: $a_1 = 9$ $a_i = a_{i-1} + 6$ What is $a_{14}$, the fourteenth term in the sequence?
Explanation: From the given formula, we can see that the first term of the sequence is $9$ and the common difference is $6$ To find the fourteenth term, we can rewrite the given recurrence as an explicit formula. The general form for an arithmetic sequence is $a_i = a_1 + d(i - 1)$ . In this case, we have $a_i = 9 + 6(i - 1)$ To find $a_{14}$ , we can simply substitute $i = 14$ into the our formula. Therefore, the fourteenth term is equal to $a_{14} = 9 + 6 (14 - 1) = 87$.